p21 (a downstream effector of p53) is believed to function as tumor suppressor. The p21 signal transduction pathway, including p53, p21, cyclin/CDK complex, p300, CBP, NFkB, Sp1, SP3, Notch 1 and PCNA, has been implicated in cell cycle control and apoptosis. Therefore, this pathway is an established target for cancer therapy (reviewed by Dotto (2000) Biochim. Biophys. Acta 1471:M43–M56). p21 has also been implicated in keratinocyte differentiation (Topley et al. (1999) Proc. Nat. Acad. Sci. USA 96:9089; Missero et al (1995) Proc. Nat. Acad. Sci. USA 92:5451; Missero et al. (1996) Genes & Devel. 10:3065; Prowse et al (1997)J. Biol. Chem. 272:1308; Di Cunto et al. (1998) Science 280:1069; Rangarajan et al. (2001) EMBO J. 20:3427; Santini et al. (2001) Proc. Nat. Acad. Sci. USA 98:9575).